1. Field of the Invention
The present invention relates to a lining material for glass melting furnaces, which is composed mainly of platinum or platinum alloy, a process for purifying the lining material, a glass melting furnace lined with the lining material, and a process for producing glass products.
2. Description of the Related Art
Glass is defective if it contain stones, cords, and bubbles. Stones are crystalline bodies of minerals, and they adversely affect the transparency and strength of glass. Cords are streaky amorphous bodies, and they adversely affect the refractive index of glass, distorting the image passing through glass. Bubbles also adversely affect the transparency of glass. Particularly, bubble wreckage on the surface of an LCD (liquid crystal display) substrate causes open circuits. Therefore, LCD glass plates and optical lenses should be free of the above-mentioned defects.
It is known that stones result from fine powder of refractory material when glass is melted in a melting furnace lined with refractory material, that cords result from refractory material dissolved in glass, and that bubbles result from refractory material in contact with glass. One way to prevent stones and cords is by lining with platinum or platinum alloy. One way to prevent bubbles is by incorporation of a fining agent into raw glass materials.
The above-mentioned measures are successful to a certain extent. Even a few bubbles are not tolerable for electronic glass (such as LCD glass). Complete elimination of bubbles is impossible even in the case of lining with industrial platinum of high purity (99.95 wt %), with the remainder (0.05 wt %) being unavoidable impurities. Countermeasures against bubbles are indispensable for further improvement in glass quality.
Despite their thorough investigation into production facilities involved, the present inventors were unable to pinpoint the source of bubbles. They assumed that the major source of bubbles is platinum in contact with molten glass.
It was found that industrial platinum in actual use varies in residual impurities depending on its origin and production lot and on whether or not it contains recycled platinum. An example of analytical data is given below.
Palladium (Pd) is a stable element and is unlikely to cause bubbles. Rhodium (Rh) is an element added to increase the strength of platinum and is also unlikely to cause bubbles. Osmium (Os) is liable to oxidation, giving rise to an oxide which vaporizes at a low temperature. In view of this and its high content, osmium is very likely to cause bubbles. Iridium (Ir) is oxidized at 800xc2x0 C. and above (although not so easily as osmium) and vaporizes at 1000xc2x0 C. and above. However, in view of the fact that iridium has long been used as a material for glass melting crucibles, iridium is unlikely to cause bubbles. Arsenic (As) vaporizes easily or oxidizes easily to form volatile oxides. However, it readily dissolves in glass and hence is unlikely to cause bubbles.
Accordingly, the present inventors have experimentally studied the relation between the amount of osmium and the number of bubbles. It turned out that the lower the content of osmium, the less the number of bubbles. This finding led to the present invention.
According to a first aspect of the present invention, there is provided a lining material for glass melting furnaces, which comprises platinum or platinum alloy as a base material containing osmium as an impurity in an amount no more than 20 ppm.
Limiting the content of osmium as above is effective in reducing the number of bubbles evolving from the lining material and entering the molten glass. For high-quality glass, the content of osmium should preferably be no more than 10 ppm.
According to a second aspect of the present invention, there is provided a glass melting furnace which is lined, at least at a part thereof in contact with molten glass, with a lining material for glass melting furnaces. The lining material comprises platinum or platinum alloy as a base material containing osmium as an impurity in an amount no more than 20 ppm.
Lining the glass melting furnaces with the lining material containing osmium as an impurity in an amount no more than 20 ppm reduces the number of bubbles entering the molten glass. For high-quality glass, the content of osmium should preferably be no more than 10 ppm.
In one preferred form, the glass melting furnace comprises a melting tank in which glass raw materials are melted, a refining tank in which molten glass undergoes a fining treatment, and a forming unit in which molten glass is drawn and formed into sheet glass. At least the forming unit is lined with the lining material. The forming unit in which molten glass is formed into sheet glass should be free of bubbles. Since the lining material does not evolve bubbles, the forming unit permits the production of a bubble-free, high-quality sheet glass.
In another preferred form, the glass melting furnace further comprises a rear facility for homogenizing, mixing, and forming defoamed molten glass. The rear facility may also be lined with the lining material.
Desirably, the refining tank is also lined at least partially with the lining material.
According to a third aspect of the present invention, there is provided a process for producing glass products, which comprises the steps of: refining molten glass in a refining tank; homogenizing the molten glass from the refining tank at a predetermined uniform temperature in a homogenizing tank; and forming the molten glass from the homogenizing tank into a sheet glass in a forming unit. The refining tank is lined, at least at a wall thereof in contact with the molten glass, with a lining material for glass melting furnaces, which material comprises platinum or platinum alloy as a base material containing osmium as an impurity in an amount no more than 20 ppm, so that the wall evolves no bubbles during refining. The homogenizing tank is lined, at least at a wall thereof in contact with the molten glass, with a lining material for glass melting furnaces, which material comprises platinum or platinum alloy as a base material containing osmium as an impurity in an amount no more than 20 ppm, so that the wall evolves no bubbles during homogenization. The forming unit is lined, at least at a wall thereof in contact with the molten glass, with a material for glass melting furnaces, which material comprises platinum or platinum alloy as a base material containing osmium as an impurity in an amount no more than 20 ppm, so that the wall evolves no bubbles during forming.
Bubble-free high-quality glass products can be obtained since refining, homogenizing, and forming steps are performed in the facilities lined with the lining material containing osmium in an amount no more than 20 ppm.
In one preferred form, the process further comprises the step of melting glass in a melting tank prior to the refining step. The melting tank may desirably be lined, at least at a wall thereof in contact with the molten glass, with a lining material for glass melting furnaces, which material comprises platinum or platinum alloy as a base material containing osmium as an impurity in an amount no more than 20 ppm, so that the wall evolves no bubbles during melting.
In another preferred form, the process further comprises the step of mixing the molten glass in a mixing tank prior to the forming step. The mixing tank is desirably lined, at least at a wall thereof in contact with the molten glass, with a lining material for glass melting furnaces, which material comprises platinum or platinum alloy as a base material containing osmium as an impurity in an amount no more than 20 ppm, so that the wall evolves no bubbles during mixing.
Since each step of the process is carried out in the facilities lined with the lining material containing osmium in an amount no more than 20 ppm, bubble-free, high quality glass products can be obtained. More desirably, the osmium content in the lining material is reduced to less than 10 ppm so that glass products of higher quality can be obtained.
According to a fourth aspec of the present invention, there is provided a process for purifying a lining material for glass melting furnaces, which comprises the steps of providing osmium-containing platinum or platinum alloy, and keeping the osmium-containing platinum or platinum alloy at a temperature higher than a temperature at which osmium oxide vaporizes, for a predetermined period of time, so that the content of osmium decreases to 20 ppm or less at least in a surface or its vicinity of the platinum or platinum alloy.
The osmium content exceeding 20 ppm can be reduced by keeping the platinum or platinum alloy at 1700xc2x0 C. for hundreds of hours.